1. Field of the Invention
The present invention relates to a composition and particularly to an inkjet ink composition for use in ink jet printing. In addition, the present invention relates to a method for manufacturing the composition, and to an image forming method using the composition.
2. Discussion of the Background
As having been disclosed in published Japanese Patent Applications Nos. 63-51485, 63-56575 and 01-198671, inkjet inks, which are prepared by dissolving one or more water-soluble dyes in water or a mixture solvent including water and a water-soluble solvent and optionally adding an additive to the solution, have been mainly developed and used. However, these inkjet inks (hereinafter referred to as dye type inkjet inks) have the following drawbacks:    (1) recorded images have so poor water resistance as to be blurred when contacting water; and    (2) recorded images have poor light resistance such that when the recorded images are exposed to light, the color tones of the images change or the image density thereof decreases.
In attempting to solve these problems of the dye type inkjet inks, pigment type inkjet inks, which use one or more pigments (e.g., carbon black and organic pigments) instead of the dyes, have been disclosed in published Japanese Patent Applications Nos. 57-10660, 57-10661 and 02-255875.
When pigment type inkjet inks are used for inkjet printing, the resultant images, which have dried on a receiving material, have good water resistance. Therefore when the images contact water, the images do not blur (i.e., the images have good water resistance). This is because pigments do not dissolve in water. In addition, since the reactivity of pigments with light is lower than that of dyes, the images formed of pigment type inks have better light resistance than those formed of dye type inks.
Such pigment type inkjet inks are typically prepared by the following method:    (1) a mixture including one or more pigments, a dispersing medium and a dispersant is subjected to a dispersing treatment using a dispersing machine such as ball mills and sand mills; and    (2) the thus prepared pigment dispersion is optionally mixed with one or more additives to prepare a pigment type inkjet ink.
When the pigment dispersion for use in an inkjet ink includes pigment particles having a relatively large particle diameter, the nozzles from which the ink is to be discharged are clogged with the large pigment particles included in the resultant inkjet ink, resulting occurrence of a problem in that the ink is not discharged from the nozzles (hereinafter referred to as a clogging problem). In addition, when large pigment particles are present in the inkjet ink, image qualities of the recorded images, such as clearness, resolution and color reproducibility (i.e., transparency) deteriorate. Therefore, the pigment dispersion is typically prepared such that the pigment particles dispersed therein have a particle diameter not greater than 200 nm to prevent the clogging problem and to produce images having good clearness, color reproducibility (in particular, color reproducibility of secondary colors (i.e., overlaid color images)) and transparency.
Recently, self-dispersion type pigments which are prepared by chemically reacting the surface of a pigment with a specific functional group which can dissociate in water and which can be dispersed without using a dispersant have been practically used.
Since a specific group such as —COONa and —SO3Na is chemically reacted with the surface of a pigment in such self-dispersion pigments, the self-dispersion pigments can be stably dispersed in water without a dispersant due to electrostatic repulsion of the functional group dissociated in water.
Black pigments such as carbon black, and yellow (Y), magenta (M) and cyan (C) color pigments such as Pigment Yellow 74, Pigment Red 122 and Pigment Blue 15:4 are preferably applied to the self-dispersion pigments.
However, both pigment type inkjet inks which use a pigment dispersion including a dispersant and pigment type inkjet inks which use a pigment dispersion including a self-dispersion pigment have a drawback in that the pH of the inks greatly changes (i.e., decreases) when the inks are preserved at 70° C. in a closed place.
If the degree of decrease in pH of an ink is large, a problem in that metal elements of an inkjet printer which contacts the inkjet ink and which are designed so as to be suitable for the original inkjet ink dissolve in or are corroded by the pH-changed inkjet ink occurs.
Because of these reasons, a need exists for a pigment type inkjet ink capable of producing good images without causing problems such as the clogging problem and pH changing problem even when used and preserved under various conditions.